Preheater for lime kiln

ABSTRACT

A preheater for particulate material comprising a cylindrical refractory lined vessel with top feed and exhaust offtake. The preheater diameter is enlarged below the offtake so that the sloping material forms an area surrounded by ports which receive hot gases from the kiln. The lower portion of the preheater is conical, leading toward a small central discharge point. This portion constitutes a soaking zone in which heat is allowed to penetrate the material.

United States Patent 1 Parsons [4 1 Oct. 23, 1973 [54] PREHEATER FORLIME KILN 2,739,800 3/1956 Sisco 263/29 X [75] Inventor: Marshall F.Parsons Washington 1,778,281 10/1930 Sobek 263/29 D.C. PrimaryExaminer-John J. Camby [73] Assignee: Detroit Lime Company, Detroit, AttKing Harness et a],

Mich.

[22] Filed: Mar. 7, 1972 57 ABSTRACT PP N05 232,579 A preheater forparticulate material comprising a cylindrical refractory lined vesselwith top feed and ex- 52] U.S. Cl. 34/168, 432/106 hahst The Preheaterdiameter is enlarged 51 Int. Cl. F26b 17/12 below the Qfftake so thatthe sloping material forms an [58] Field of Search 263/29, 32; 432/106;area shmhhded by Ports which receive gases from 34/168 the kiln. Thelower portion of the preheater is conical, leading toward a smallcentral discharge point. This [56] References Cited portion constitutesa soaking zone in which heat is al- UNTED STATES PATENTS lowed topenetrate the material.

2,653,809 9/1953 Azbe 263/32 R 13 Claims, 3 Drawing Figures II I.i\\\\\- i r i j 7/ Patented Oct. 23, 1973 2 Sheets-Sheet 2 PREIIEATERFOR LIME KILN BACKGROUND AND BRIEF SUMMARY OF THE INVENTION Theinvention relates to the preheating of stone or sized particulatematerial such as limestone which is being fed to a rotary kiln calciningsystem. The invention is also applicable to similar calcining orgas/solid heat transfer processes.

A rotary kiln calcining device has relatively low thermal efficiency,particularly with highly endothermic processes such as the burning oflime. Rising fuel costs have increased the economic feasibility ofpreheating devices for commercial rotary kilns.

These preheating devices are based'on convection heat transfer with thehot gases exhausting from the kiln being passed through voids in thefeed material as in a vertical'shaft kiln. Because of the relativelysmall size of rotary kiln feeds and the large volume of combustiongases, impractically high pressure drops will be developed in the systemunless the beds are relatively wide and shallow. The gases should haveuniform motion across the entire bed to avoid overheating or fus-Because of the relatively higher heat transfer efficiency of preheaters,the calcining reaction tends to move from the rotary kiln into thepreheater resulting in higher kiln exhaust gas temperatures. These'hightemperature gases-result in high surface temperatures on the stone feedfed to the kiln, with the result that the preheater has little coolingeffect on the gases. The cumulative effect is that a circulating heatstream is set up at the juncture of the preheater and kiln outlet, the,high temperatures increasing heat losses and mechanical difficulties. v

In the conventional preheater the kiln exhaust gases 'are introduceddirectly to the preheater discharge area.

The resulting high temperatures create great radiation and efficiencylosses. A cumbersome feeding mechanism is required to be effective overthe majorportion of the bed area, this mechanism operating at unusuallyhigh temperatures. Furthermore, the short time interval during which thefeed material is subjected to high temperatures results in only theouter surface portions of the stone being heated, the centers remainingat comparatively low temperatures.

It is a generalobject of thepresent invention to overcome the abovedifficulties of conventional preheating devices. Briefly, the'inventioncomprises a'moving'bed of material so arranged that the kiln exhaustgases are introduced at a level in the preheater considerably above thefeeder which discharges the stone to the rotary kiln. This creates asoaking or holding zone between the gas entry and discharge levels whichmay have a sufficient retention time to allow the high surfacetemperatures developed on each particle to'be diffused substantiallyinto its entire volume. The result "is a preheated material of moreuniform'temperature with a lower surface temperature but a greaterstored heat content. When this'material is'fed to the rotary kiln itwill not have the tendency to return heat to the gas stream but willabsorb some heat, maintaining gas temperatures at a lower level.

It is a further object of the invention to provide a novel and improvedpreheater in which the soaking zone portion couples the'entirecross-sectional area of the preheaterstone beds toa relatively small andsimple feeding device working at moderate temperatures, thus reducingmaintenance problems. The conically formed holding zone contributes tothe uniform motion of the material and facilitates coupling the largepreheater area to the single feeder.

It is a further object to provide an improved preheater of this naturewhich develops sufficient stone bed surface to allow admission of thegases into the relatively small void areas of the material. The hotgases are introduced around the outer periphery of a circularcross-section and are removed from the center. Since flow resistance ishigher for hot than for cooler gases,

this arrangement allows the development of stone bed void areas roughlyproportional to the gas temperatures to give efiicient use of the bedarea.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view inelevation of the novel preheater taken along the line l1 of FIG. 2;

FIG. 2 is a plan crosssectional view taken along the line 2-2 of FIG. 1;and

FIG. 3 is a fragmentary cross-sectional view taken along the line 33 ofFIG. 2 and showing louver construction.

DESCRIPTION OF THE PREFERRED EMBODIMENT The preheater is generallyindicated at 11 and in general comprises a vessel having an outer steelshell or framework generally indicated at 12 and an inner lininggenerally indicated at 13 of refractory material. Since the inventionpertains to the configuration rather than the specific structuraldetails of the preheater, the individual portions of the shell,framework and lining are not described in full detail. It will beunderstood however that appropriate supports, braces, refractorylinings, poke and blow holes and other structural elements may be variedto suit requirements.

The stone beds 14 are formed in a cylindrically shaped section 15of'this vessel. An exhaustofftake 16 is concentrically mounted insection 15 and is of downwardly open, flared shape having an outerdiameter of perhaps 60 to percent of the vessel diameter. The offtake isshown as supported by a horizontal duct 17 extending through section 15and leading to a blower intake (not shown). The top of vessel section 15is enclosed by an air lock feed device indicated schematically indot-dash lines at 18 through which the stone is introduced, so that thekiln exhaust gases may be drawn by induced draft through the stonebedswithout drawing in cool ambient air. The air lock feed device may takeany of various conventional forms,'such as a rotating gate having aplurality ofpockets formed by radial vanes. The upper level 19 ofmaterial is maintained at a point above exhaustofftake 16 so that thematerial forms a symmetrical stone volume for exhaust of the gases.

At a suitable depth below the level of offtake 16 the cylindricalpreheater walls are increased to a larger diameter as indicated by thereference numeral 20. The preheater has a horizontal ledge 21 whichinterconnects wall sections 15 and 20 and extends outwardly beyondsection 20. A wall 22 extends downwardly from the outer edge of ledge21, thus creating an annular distribution duct 23 between vessel section20 and wall 22. Exhaust gases 24 from the rotary kiln (not shown) arefed upwardly through a conduit 25 to this duct. Be-

cause of the angle of repose of frustoconical surface 26 of stone bed 14as it leaves section 15 and spreads out to section 20, an annular space27 will be created below ledge 21. A plurality of ports 28 are providedin the upper edge of section immediately below ledge Zl, and hot gaseswill be drawn inwardly from duct 23 through these ports to the stonebed. The flow of hot gases as they pass through the ports will begenerally in an inward radial direction, and transverse to the downwardmovement of material. The hot gases will thus be evenly distributed overthe relatively large frustoconical area of the material which issymmetrical with respect to the offtake area. The gases will flowupwardly through the voids in the material as they pass through section15 to the offtake. Duct 23 will allow dust entrained in the gases tosettle, the dust being removable through drains 29.

A soaking or holding chamber 30 is formed on the preheater extendingbelow section 20. This portion of the preheater is formed conically at arelatively steep angle, leading downwardly to a single centrally locateddischarge port 31 of relatively small area. A feeding device indicatedschematically in dot-dash lines at 32 is attached to outlet 31 anddispatches the material through a conventional feed pipe or chuteindicated in dot-dash lines at 33 to the rotary kiln (not shown). Toassure movement of the material over the entire crosssection of thepreheater, a baffle 34 is mounted inwardly of conical section 30, thisbaffle having a conical shape along the major portion of its lengthwhich is complementary to section 30 and a reverse conical shape 35 inits upper portion. The baffle, which is supported by a plurality ofradially extending beams 36, will thus insure movement of the materialover the entire cross-section of preheater section 30 to outlet 31.Little gas flow will take place in this zone because of the highresistance of the deep beds and the relatively small cross-sectionalarea, and the section will thus constitute a holding zone in which heatwill be absorbed by the interiors of the particles. i

The dimensions of holding section 30 and particularly the retention timeof stone therein, will vary to suit requirements. These will be governedby three main factors:

a. Temperature of the gases entering section 20.

b. Average size of the stone particles.

c. Thermal conductivity of the stone.

Basically, the stone which has just exited gas-stone section 20 has asurface temperature equal to the gas temperature. Let us assume forexample that it is 2,000 F on the surface of a 1% inch spherical lump,but only l,200 F at the center. At a thermal conductivity in theexpected range of slate or shale (for lightweight aggregate kilns) thethermal diffusion might almost equalize the temperature over the outerthreeeighths inch (or seven-eighths of the total lump volume) in 7 to 11minutes.

In limestone there is some difference in that there is heat used in thecontact zone to make a thin film of lime on the stone surface, so thatthe inside of the stone will not have absorbed as much heat. As thispasses through holding section 30, the surface may recarbonate makinglimestone and giving off heat. This will delay the achievement ofrelative equilibrium-and require a longer time in the holding section.

In operation, hot gases 24 will flow upwardly through conduit 25 andaround duct 23, entering chamber 27 through ports 28 and passing throughthe voids in material 14. These gases will flow upwardly throughcylindrical section 15 of the preheater to exhaust offtake 16. As thematerial moves downwardly through this zone, it will spread out againstvessel section 20, this section, like section 15, being an activeheating area in which the hot gases flow through the material voids.Because the heating area is relatively close to feed device 18 throughwhich the particles enter at ambient temperatures, there will be largetemperature differentials between the hot gases and the surfaces of theparticles. This will result in immediately high rates of heatabsorption, contributing to higher production for a given amount offuel, or less fuel per unit of productivity.

As the heated material continues to descend, it will enter holdingsection 30 where it will gradually be narrowed down to a relativelysmall cross-sectional area so as to be discharged at central opening 31.During the time the material is in the holding zone, the heat absorbedby the outer portions of the particles will soak into their interiors tocreate a more uniform temperature throughout.

I claim:

1. In a preheater for particulate materials such as limestone, arefractory lined vessel having an upper material intake section, anexhaust offtake concentrically mounted in said section, means forfeeding material into said section, a gas intake section below saidmaterial intake section, a plurality of circumferentially arranged portsaround said gas intake section, means feeding hot gases to said ports,whereby the gases will flow radially inwardly through said material, amaterial holding section extending downwardly from said gas intakesection, said material holding section being entirely below said portsand being sufficiently deep to permit heat to be absorbed by theinteriors of the particles of material, and a material discharge port atthe bottom of said material holding section.

2. The combination according to claim 1, said material intake sectionbeing cylindrical, said exhaust offtake being circular, said materialfeeding means being so arranged that the level of material is maintainedabove said offtake. 4 A

3. The combination according to claim 1, said gas intake section beingof larger diameter than said material intake section and so shaped thatthe angle of repose of descending material will form an inclinedfrustoconical surface around its periphery.

4. The combination according to claim 3, said gas feeding meanscomprising an annular distribution duct around said ports, the portsbeing located adjacent the top of said enlarged section.

5. The combination according to claim 1, said material holding sectionbeing conical, said material discharge port being centrally located andof relatively small area.

6. The combination according to claim 5, further provided with a conicallower baffle concentric with and spaced inwardly from said conicalholding member, whereby the entire cross-sectional area of said holdingzone will be coupled to said discharge port.

7. In a preheater for particulate materials such as limestone, arefractory lined vessel having a cylindrical section, a circular exhaustofftake concentrically mounted in said section, means for feedingmaterial into the top of said vessel so that the level of material ismaintained above said offtake, a section of larger diameter than saidcylindrical vessel section below the offtake and so shaped that theangle of repose of descending material will form an outwardly anddownwardly inclined frustoconical surface around its periphcry, aplurality of circumferentially arranged ports adjacent the top of saidenlarged section, means feeding hot gases to an annular distributionduct around said ports, a conical material holding section extendingdownwardly from said enlarged section, said material holding sectionbeing entirely below said ports and being sufficiently deep to permitheat to be absorbed by the interiors of the particles of material, and asingle centrally located material discharge port of relatively smallarea at the bottom of said holding section.

8. The combination according to claim 7, further provided with a conicallower baffle concentric with and spaced inwardly from said conicalholding chamber, whereby the entire crosssectional area of said holdingzone will be coupled to said discharge port.

9. The combination according to claim 8, said offtake comprising adownwardly open frustoconical member connected to a horizontal ductpassing through said cylindrical vessel section.

10. In a preheater for particulate materials such as limestone, arefractory lined vessel having a cylindrical section, a circular exhaustofftake concentrically mounted in said section, means for feedingmaterial into the top of said vessel so that the level of material ismaintained above said offtake, a section of larger diameter than saidcylindrical vessel section below the offtake and so shaped that theangle of repose of descending material will form an inclinedfrustoconical surface around its periphery, a plurality ofcircumferentially arranged ports adjacent the top of said enlargedsection, means feeding hot gases to an annular distribution duct aroundsaid ports, said annular distribution duct being so formed as to allowsettling of dust entrained in said gases, drains for said dust in thebottom of said duct, a conical material holding section extendingdownwardly from said enlarged section, and a single centrally locatedmaterial discharge port of relatively small area at the bottom of saidholding section.

11. In a preheater for particulate materials such as limestone, arefractory lined vessel having an upper material intake section, anexhaust offtake in said section, means for feeding material into saidsection, a gas intake section, means feeding hot gases to said gasintake section, a material holding section extending downwardly fromsaid gas intake section, and a material discharge port at the bottom ofsaid material holding section, said gas intake section being located asubstantial distance above said discharge port whereby the hot gaseswill flow across stone having a relatively low surface temperature, saidmaterial holding section being entirely below said gas intake sectionand of sufficient dimensions to allow time for substantial inwarddiffusion of the surface heat on particles descending therethrough.

12. The combination according to claim 11, said material intake sectionbeing cylindrical, said material holding section being conical.

13. The combination according to claim 12, said gas intake section beingdisposed between said material intake and holding sections and having anannular gas distribution duct.

1. In a preheater for particulate materials such as limestone, arefractory lined vessel having an upper material intake section, anexhaust offtake concentrically mounted in said section, means forfeeding material into said section, a gas intake section below saidmaterial intake section, a plurality of circumferentially arranged portsaround said gas intake section, means feeding hot gases to said ports,whereby the gases will flow radially inwardly through said material, amaterial holding section extending downwardly from said gas intakesection, said material holding section being entirely below said portsand being sufficiently deep to permit heat to be absorbed by theinteriors of the particles of material, and a material discharge port atthe bottom of said material holding section.
 2. The combinationaccording to claim 1, said material intake section being cylindrical,said exhaust offtake being circular, said material feeding means beingso arranged that the level of material is maintained above said offtake.3. The combination according to claim 1, said gas intake section beingof larger diameter than said material intake section and so shaped thatthe angle of repose of descending material will form an inclinedfrustoconical surface around its periphery.
 4. The combination accordingto claim 3, said gas feeding means comprising an annular distributionduct around said ports, the ports being located adjacent the top of saidenlarged section.
 5. The combination according to claim 1, said materialholding section being conical, said material discharge port beingcentrally located and of relatively small area.
 6. The combinationaccording to claim 5, further provided with a conical lower baffleconcentric with and spaced inwardly from said conical holding member,whereby the entire cross-sectional area of said holding zone will becoupled to said discharge port.
 7. In a preheater for particulatematerials such as limestone, a refractory lined vessel having acylindrical section, a circular exhaust offtake concentrically mountedin said section, means for feeding material into the top of said vesselso that the level of material is maintained above said offtake, asection of larger diameter than said cylindrical vessel section belowthe offtake and so shaped that the angle of repose of descendingmaterial will form an outwardly and downwardly inclined frustoconicalsurface around its periphery, a plurality of circumferentially arrangedports adjacent the top of said enlarged section, means feeding hot gasesto an annular distribution duct around said ports, a conical materialholding section extending downwardly from said enlarged section, saidmaterial holding section being entirely below said ports and beingsufficiently deep to permit heat to be absorbed by the interiors of theparticles of material, and a single centrally located material dischargeport of relatively small area at the bottom of said holding section. 8.The combination according to claim 7, further provided with a conicallower baffle concentric with and spaced inwardly from said conicalholding chamber, whereby the entire crosssectional area of said holdingzone will be coupled to said discharge port.
 9. The combinationaccording to claim 8, said offtake comprising a downwardly openfrustoconical member connected to a horizontal duct passing through saidcylindrical vessel section.
 10. In a preheater for particulate materialssuch as limestone, a refractory lined vessel having a cylindricalsection, a circular exhaust offtake concentrically mounted in saidsection, means for feeding material into the top of said vessel so thatthe level of material is maintained above said offtake, a section oflarger diameter Than said cylindrical vessel section below the offtakeand so shaped that the angle of repose of descending material will forman inclined frustoconical surface around its periphery, a plurality ofcircumferentially arranged ports adjacent the top of said enlargedsection, means feeding hot gases to an annular distribution duct aroundsaid ports, said annular distribution duct being so formed as to allowsettling of dust entrained in said gases, drains for said dust in thebottom of said duct, a conical material holding section extendingdownwardly from said enlarged section, and a single centrally locatedmaterial discharge port of relatively small area at the bottom of saidholding section.
 11. In a preheater for particulate materials such aslimestone, a refractory lined vessel having an upper material intakesection, an exhaust offtake in said section, means for feeding materialinto said section, a gas intake section, means feeding hot gases to saidgas intake section, a material holding section extending downwardly fromsaid gas intake section, and a material discharge port at the bottom ofsaid material holding section, said gas intake section being located asubstantial distance above said discharge port whereby the hot gaseswill flow across stone having a relatively low surface temperature, saidmaterial holding section being entirely below said gas intake sectionand of sufficient dimensions to allow time for substantial inwarddiffusion of the surface heat on particles descending therethrough. 12.The combination according to claim 11, said material intake sectionbeing cylindrical, said material holding section being conical.
 13. Thecombination according to claim 12, said gas intake section beingdisposed between said material intake and holding sections and having anannular gas distribution duct.